Based on Agatha Christie's "The Secret Adversary," a young couple looking for adventure become involved in the mystery of a missing woman and an espionage ring.

During a railway journey, Baron van Geldern meets Evelyne, an attractive young woman travelling in the same carriage. After the train is derailed, they spend a night together at a hotel, and the womanizer falls in love with her. Complications ensue when he discovers that she is engaged to his American friend.